Method for the catalytic gas phase oxidation of propene into acrolein

ABSTRACT

In a process for the catalytic gas-phase oxidation of propene to acrolein, the reaction gas starting mixture is passed with a propene loading of &gt;=160 l(S.T.P.)/l.h over a fixed-bed catalyst which is housed in two spatially successive reaction zones A,B, the reaction zone B being kept at a higher temperature than the reaction zone A.

The present invention relates to a process for the catalytic gas-phaseoxidation of propene to acrolein, in which a reaction gas startingmixture comprising propene, molecular oxygen and at least one inert gas,at least 20% by volume of which consists of molecular nitrogen, andcontaining the molecular oxygen and the propene in a molar ratio O₂:C₃H₆of ≧l is passed over a fixed-bed catalyst, whose active material is atleast one molybdenum- and/or tungsten- and bismuth-, tellurium-,antimony-, tin- and/or copper-containing multimetal oxide, in such a waythat the propene conversion in a single pass is ≧90 mol % and theassociated selectivity of the acrolein formation and of the acrylic acidbyproduct formation together is ≧90 mol %.

The abovementioned process for the catalytic gas-phase oxidation ofpropene to acrolein is generally known (cf. for example EP-A 15565, EP-A700714, DE-C 2830765, DE-C 3338380, JP-A 91/294239, EP-A 807 465, WO98/24746, EP-B 279374, DE-C 2513405, DE-A 3300044, EP-A 575897 and DE-A19855913) and is important in particular as the first oxidation stage inthe preparation of acrylic acid by two-stage catalytic gas-phaseoxidation of propene in two reaction stages in series (cf. for exampleDE A 3002829). Acrylic acid is an important monomer which is used assuch or in the form of its alkyl esters for the production of polymerssuitable, for example, as adhesives.

Since a small amount of acrylic acid byproduct is usually formed in theabovementioned catalytic gas-phase oxidation of propene to acrolein and,according to the above, acrylic acid is as a rule the desired naturalsecondary product of acrolein, the molar sum of acrolein formed andacrylic acid formed as byproduct is usually considered as desiredproduct in a catalytic gas-phase oxidation of propene to acrolein. Thisapproach is also to be applicable in the present patent application.

The object of any catalytic fixed-bed gas-phase oxidation of propene toacrolein is in principle to obtain a very high space-time yield (STY) ofdesired product (in a continuous procedure, this is the amount ofdesired product produced in grams per hour and unit volume of thecatalyst bed used in liters).

There is therefore general interest in carrying out the gas-phaseoxidation with a very high loading of the catalyst bed with propene(this is understood as meaning the amount of propene in liters understandard temperature and pressure conditions (=l(S.T.P.); the volume inliters which the corresponding amount of propene would assume understandard temperature and pressure conditions, i.e. at 25° C. and 1 bar)which is passed as a component of the reaction gas mixture, per hour,through one liter of catalyst bed), without significantly impairing thepropene conversion taking place in a single pass of the reaction gasstarting mixture through the catalyst bed and the selectivity of theassociated formation of desired product.

The implementation of the abovementioned is impaired by the fact thatgas-phase oxidation of propene to acrolein on the one hand is highlyexothermic and on the other hand is accompanied by a multiplicity ofpossible parallel and subsequent reactions.

With increasing loading of the catalyst bed with propene, and withrealization of the desired boundary condition of an essentially constantpropene conversion, it must therefore be assumed that the selectivity ofthe formation of desired product decreases as a result of the greaterheat production (cf. EP-B 450596, Example 1 and 2).

The conventional processes for the catalytic gas-phase oxidation ofpropene to acrolein, wherein nitrogen is used as a main component of theinert diluent gas and in addition a fixed-fed catalyst present in areaction zone and homogeneous along this reaction zone, i.e. having achemically uniform composition over the catalyst bed, is employed andthe temperature of the reaction zone is kept at a value standard overthe reaction zone (temperature of the reaction zone here is understoodas meaning the temperature of the catalyst bed present in the reactionzone when the process is carried out in the absence of a chemicalreaction; if this temperature in such reaction zone is not constant, theterm temperature of the reaction zone means here the numerical averageof the temperature of the catalyst bed along the reaction zone),therefore limit the applicable value of the propene loading of thecatalyst bed to ≦155 l(S.T.P.) of propene per l of catalyst bed per h(cf. for example EP-A 15565 (maximum propene load=120 l(S.T.P.) ofpropene/l·h), DE-C 2830765 (maximum propene load=94.5 l(S.T.P.) ofpropene/l·h), EP-A 804465 (maximum propene load=128 l(S.T.P.) ofpropene/l·h), EP-B 279374 (maximum propene load=112 l(S.T.P.) ofpropene/l·h), DE-C 2513405 (maximum propene load=110 l(S.T.P.) ofpropene/l·h), DE-A 3300044 (maximum propene load=112 l(S.T.P.) ofpropene/l·h), EP-A 575897 (maximum propene load=120 l(S.T.P.) ofpropene/l·h), DE-C 3338380 (in essentially all examples, the maximumpropene load is 126 l(S.T.P.) of propene/l·h; only in the case of aspecial catalyst composition was a propene load of 162 l(S.T.P.)/l·hrealized) and DE-A 19855913 (maximum propene load=155 l(S.T.P.) ofpropene/l·h)).

WO 98/24746 considers it necessary, even at a propene loading of up to148.8 l(S.T.P.) of propene/l·h, to structure the catalyst bed in such away that its volume-specific activity increases gradually in thedirection of flow of the reaction gas mixture.

Although JP-A 91/294239 discloses, in an exemplary embodiment, that apropene load of the catalyst bed with 160 l(S.T.P.) of propene/l·h ispossible in an essentially conventional procedure for a catalyticgas-phase oxidation of propene to acrolein, this is likewise only at theexpense of a volume-specific activity gradually increasing in thedirection of flow of the reaction gas mixture. However, such a procedureis not very practicable on an industrial scale, the gas-phase catalyticoxidation of propene to acrolein usually being carried in tube-bundlereactors comprising a few thousand catalyst tubes, each individual oneof which has to be loaded with a gradated catalyst bed.

EP-B 253409 and the associated equivalent, EP-B 257565, disclose that,with the use of an inert diluent gas which has a higher molar heatcapacity than molecular nitrogen, the propene content of the reactiongas starting mixture can be increased. Nevertheless, in the twoabovementioned publications, the maximum realized propene loading of thecatalyst bed is 140 l(S.T.P.) of propene/l·h.

Only in EP-A 293224 have propene loadings above 160 l(S.T.P.) ofpropene/l·h been realized to date. However, this has been achieved atthe expense of a special inert diluent gas to be used, which iscompletely free of molecular nitrogen. The disadvantage of this diluentgas is in particular the fact that, in contrast to molecular nitrogen,all its components are desired products which, when the process iscarried out continuously, have to be recycled at least partially to thegas-phase oxidation in an expensive manner for cost-efficiency reasons.

EP-B 450596 used a structured bed of catalyst and obtained a propeneloading of 202.5 l(S.T.P)/l·h, but at the cost of reduced selectivity todesired product.

It is an object of the present invention to provide a process, asdefined at the outset, for the catalytic gas-phase oxidation of propeneto acrolein, which process ensures a higher space-time yield withrespect to desired product without having the disadvantages of thehigh-load procedures of the prior art.

We have found that this object is achieved by a process for thecatalytic gas-phase oxidation of propene to acrolein, in which areaction gas starting mixture comprising propene, molecular oxygen andat least one inert gas, at least 20% by volume of which consists ofmolecular nitrogen, and containing the molecular oxygen and the propenein a molar ratio O₂:C₃H₆ of ≧1 is passed, at elevated temperatures, overa fixed-bed catalyst, whose active material is at least one molybdenum-and/or tungsten- and bismuth-, tellurium-, antimony-, tin- and/orcopper- (preferably at least one Mo-, Bi- and Fe-)containing multimetaloxide, in such a way that the propene conversion in a single pass is ≧90mol % and the associated selectivity of the acrolein formation and ofthe acrylic acid byproduct formation together is ≧90 mol %, wherein

a) the loading of the fixed-bed catalyst with the propene contained inthe reaction gas starting mixture is ≧160 l(S.T.P.) of propene per l ofcatalyst bed per h,

b) the fixed-bed catalyst consists of a catalyst bed arranged in twospatially successive reaction zones A, B, the temperature of thereaction zone A being from 300 to 390° C. (frequently to 350° C.) andthe temperature of the reaction zone B being from 305 to 4200° C.(frequently up to 380° C.) and at the same time being at least 5° C.above the temperature of the reaction zone A,

c) the reaction gas starting mixture flows first through the reactionzone A and then through the reaction zone B and

d) the reaction zone A extends to a propene conversion of from 40 to 80mol %.

Preferably, the reaction zone A extends to a propene conversion of from50 to 70, particularly preferably from 65 to 75, mol %.

According to the invention, the temperature of the reaction zone B isadvantageously from 305 to 365° C., or 340° C., particularlyadvantageously from 310 to 3300° C.

Furthermore, the temperature of the reaction zone B is preferably atleast 10° C. above the temperature of the reaction zone A.

The higher the chosen propene loading of the catalyst bed in the novelprocess, the greater should be the chosen difference between thetemperature of the reaction zone A and the temperature of the reactionzone B. Usually, however, the abovementioned temperature difference inthe novel process will be not more than 50° C., i.e. the differencebetween the temperature of the reaction zone A and the temperature ofthe reaction zone B may be, according to the invention, up to 20° C., upto 25° C., up to 30° C., up to 40° C., up to 45° C. or up to 50° C.

As a rule, the propene conversion, based on the one pass, in the novelprocess is ≧92 mol % or ≧94 mol %. The selectivity of the formation ofdesired product is usually ≧92 mol % or ≧94 mol %, frequently ≧95 mol %or ≧96 mol % or ≧97 mol %.

Surprisingly, the abovementioned applies not only in the case of propeneloadings of the catalyst bed of ≧165 l(S.T.P.)/l·h or of ≧170l(S.T.P.)/l·h or ≧175 l(S.T.P.)/l·h or ≧180 l(S.T.P.)/l·h, but also inthe case of propene loadings of the catalyst bed of >20 185l(S.T.P.)/l·h or ≧190 l(S.T.P.)/l·h or ≧200 l(S.T.P.)/l·h or ≧210l(S.T.P.)/l·h and in the case of loadings of ≧220 l(S.T.P.)/l·h or ≧230l(S.T.P.)/l·h or ≧240 l(S.T.P.)/l·h or >250 l(S.T.P.)/l·h.

In this respect, it is surprising that the abovementioned values areachievable even when the inert gas used according to the inventioncomprises ≧30% by volume or ≧40% by volume or ≧50% by volume or ≧60% byvolume or ≧70% by volume or ≧80% by volume or ≧90% by volume or ≧95% byvolume of molecular nitrogen. In the case of propene loadings above 250l(S.T.P.)/l·h, the presence of inert (inert diluent gases are intendedin general to be those which undergo less than 5%, preferably less than2%, conversion during a single pass) diluent gases such as propane,ethane, methane, pentane, butane, CO₂, CO, steam and/or noble gases, isrecommended for the novel process. However, these gases and theirmixtures can of course also be used in the case of lower loadings and assole diluent gases. It is furthermore surprising that the novel processcan be carried out using a catalyst bed which is homogeneous, i.e.chemically uniform, over both reaction zones, without sufferingsignificant declines in the conversion and/or in selectivity.

In the novel process, the propene loading usually will not exceed 600l(S.T.P.)/l·h. In the novel process, without significant loss ofconversion and selectivity, the propene loadings are typically ≦30,frequently ≦25, l(S.T.P.)/l·h.

In the novel process, the operating pressure may be either belowatmospheric pressure (for example up to 0.5 bar) or above atmosphericpressure. Typically, the operating pressure is from 1 to 5 bar,frequently from 1.5 to 3.5, bar. Usually, the reaction pressure will notexceed 100 bar.

According to the invention, the molar O₂:C₃H₆ ratio in the reaction gasstarting mixture must be ≧1. Usually, this ratio is ≦3. According to theinvention, the molar O₂:C₃H₆ ratio in the reaction gas starting mixtureis frequently ≧1.5 and ≦2.0.

A suitable source of the molecular oxygen required in the novel processis air as well as air depleted in molecular nitrogen (for example ≧90%by volume Of O₂ and ≦10% by volume of N₂).

According to the invention, the propene content of the reaction gasstarting mixture may be, for example, from 4 to 15, frequently from 5 to12, % by volume or from 5 to 8% by volume (based in each case on thetotal volume).

Frequently, the novel process is carried out at a volume ratio ofpropene to oxygen to inert gases (including steam) in the reaction gasstarting mixture of 1:(1.0 to 3.0):(5 to 25), preferably 1:(1.7 to2.3):(10 to 15).

Usually, the reaction gas starting mixture contains essentially nofurther components apart from said constituents.

Suitable fixed-bed catalysts for the novel process are all those whoseactive material is at least one Mo-, Bi- and Fe-containing multimetaloxide.

This means in principle that all those catalysts which are disclosed inDE-C 3338380, DE-A 19902562, EP-A 15565, DE-C 2830765, EP-A 807465, EP-A279374, DE-A 3300044, EP-A 575897, U.S. Pat. No. 4438217, DE-A 19855913,WO 98/24746, DE-A 19746210 (those of the formula II), JP-A 91/294239,EP-A 293224 and EP-A 700714 can be used according to the invention. Thisapplies in particular to the exemplary embodiments in thesepublications, among which those of EP-A 15565, of EP-A 575897, of DE-A19746210 and of DE-A 19855913 are particularly preferred. A catalystaccording to Example 1c of EP-A 15565 and a catalyst which is to beprepared in a corresponding manner but whose active material has thecomposition Mo₁₂Ni_(6.5)Zn₂Fe₂Bi₁P_(0.0065)K_(0.06)O_(x).10SiO₂ areparticularly noteworthy in this context. The example with theconsecutive No. 3 from DE-A 19855913 (stoichiometry: Mo₁₂Co₇ Fe₃Bi_(0.6)K_(0.08)Si_(1.6)O_(x)) as an unsupported catalyst in the form ofhollow cylinders and measuring 5 mm×3 mm×2 mm (externaldiameter×height×internal diameter) and the unsupported multimetal oxideII catalyst according to Example 1 of DE-A 19746210 are also noteworthy.The multimetal oxide catalysts of U.S. Pat. No. 4438217 should also bementioned. The latter applies in particular when these hollow cylindersmeasure 5 mm×2 mm×2 mm or 5 mm×3 mm×2 mm or 6 mm×3mm×3mm or 7 mm×3mm×4mm (in each case external diameter×height×internal diameter).

A multiplicity of the multimetal oxide active materials suitableaccording to the invention can be subsumed under the formula I

Mo ₁₂ Bi _(a) Fe _(b) X _(c) ¹ X _(d) ² X _(e) ³ X _(f) ⁴ O _(n)  (I),

where

X¹ is nickel and/or cobalt,

X² is thallium, an alkali metal and/or an alkaline earth metal,

X³ is zinc, phosphorus, arsenic, boron, antimony, tin, cerium, leadand/or tungsten,

X⁴ is silicon, aluminum, titanium and/or zirconium,

a is from 0.5 to 5,

b is from 0.01 to 5, preferably from 2 to 4,

c is from 0 to 10, preferably from 3 to 10,

d is from 0 to 2, preferably from 0.02 to 2,

e is from 0 to 8, preferably from 0 to 5,

f is from 0 to 10 and

n is a number which is determined by the valency and frequency of theelements in I other than oxygen.

They are obtainable in a manner known per se (cf. for example DE-A4023239) and are usually molded as such into spheres, rings or cylindersor used in the form of coated catalysts, i.e. premolded inert carrierscoated with the active material. However, they can of course also beused in powder form as catalysts. According to the invention, it is ofcourse also possible to use the Bi-, Mo- and Fe-comprising multimetaloxide catalyst ACS-4 from Nippon Schokubai.

In principle, active materials suitable according to the invention, inparticular those of the formula I, can be prepared in a simple manner byproducing, from suitable sources of their elemental constituents, a veryintimate, preferably finely divided dry blend having a compositioncorresponding to their stoichiometry and calcining said dry blend atfrom 350 to 650° C. The calcination can be carried out either underinert gas or under an oxidizing atmosphere, e.g. air (mixture of inertgas and oxygen) or under a reducing atmosphere (for example, a mixtureof inert gas, NH₃, CO and/or H₂). The duration of calcination may befrom a few minutes to a few hours and usually decreases with increasingtemperature. Suitable sources of the elemental constituents ofmultimetal oxide active materials I are those compounds which arealready oxides and/or those compounds which can be converted into oxidesby heating, at least in the presence of oxygen.

In addition to the oxides, suitable starting compounds of this type arein particular halides, nitrates, formates, oxalates, citrates, acetates,carbonates, amine complexes, ammonium salts and/or hydroxides (compoundssuch as NH₄OH, (NH₄)₂CO₃, NH₄NO₃, NH₄CHO₂, CH₃COOH, NH₄CH₃CO₂ and/orammonium oxalate, which decompose and/or can be decomposed, at thelatest during the subsequent calcination, into compounds which escapecompletely in gaseous form, may additionally be incorporated into theintimate dry blend).

The thorough mixing of the starting compounds for the preparation ofmultimetal oxide materials I can be effected in dry or in wet form. Ifit is carried out in dry form, the starting compounds are expedientlyused in the form of finely divided powders and, after mixing and anycompaction, are subjected to calcination. However, the thorough mixingis preferably effected in wet form. Usually, the starting compounds aremixed with one another in the form of an aqueous solution and/orsuspension. Particularly intimate dry blends are obtained in the mixingmethod described when exclusively sources of the elemental constituentsin solution are used as starting materials. A preferably used solvent iswater. The aqueous material obtained is then dried, the drying processpreferably being carried out by spray-drying the aqueous mixture withoutlet temperatures of from 100 to 150° C.

The multimetal oxide materials suitable according to the invention, inparticular those of the formula I, can be used for the novel processboth in powder form and after molding to give specific catalystgeometries, it being possible to carry out the shaping before or afterthe final calcination. For example, unsupported catalysts can beprepared from the powder form of the active material or its uncalcinedand/or partially calcined precursor material by compaction to give thedesired catalyst geometry (for example by pelleting or extrusion), itbeing possible to add assistants, such as graphite or stearic acid aslubricants and/or molding assistants and reinforcing agents, such asmicrofibers of glass, asbestos, silicon carbide or potassium titanate.Suitable unsupported catalyst geometries are, for example, solidcylinders or hollow cylinders having an external diameter and a lengthof from 2 to 10 mm. In the case of hollow cylinders, a wall thickness offrom 1 to 3 mm is expedient. The unsupported catalyst may of course alsohave spherical geometry, it being possible for the sphere diameter to befrom 2 to 10 mm.

The shaping of the pulverulent active material or of its pulverulent,still uncalcined and/or partially calcined precursor material can ofcourse also be effected by application to premolded inert catalystcarriers. The coating of the carriers for the preparation of the coatedcatalysts is carried out, as a rule, in a suitable rotatable container,as disclosed, for example, in DE-A 2909671, EP-A 293859 or EP-A 714700.For coating the carriers, the powder material to be applied isexpediently moistened and, after application, is dried again, forexample by means of hot air. The coat thickness of the powder materialapplied to the carrier is expediently chosen to be from 10 to 1000 μm,preferably from 50 to 500 μm, particularly preferably from 150 to 250μm.

Conventional porous or nonporous aluminas, silica, thorium dioxide,zirconium dioxide, silicon carbide or silicates, such as magnesiumsilicate or aluminum silicate, can be used as carrier materials. Thecarriers may have a regular or irregular shape, those having a regularshape and substantial surface roughness, for example spheres or hollowcylinders, being preferred. The use of essentially nonporous, sphericalsteatite carriers which have a rough surface and whose diameter is from1 to 8 mm, preferably from 4 to 5 mm, is suitable. However, the use ofcylinders whose length is from 2 to 10 mm and whose external diameter isfrom 4 to 10 mm as carriers is also suitable. In the case of ringssuitable according to the invention as carriers, the wall thickness ismoreover usually from 1 to 4 mm. Annular carriers preferably to be usedaccording to the invention have a length of from 3 to 6 mm, an externaldiameter of from 4 to 8 mm and a wall thickness of from 1 to 2 mm. Ringsmeasuring 7 mm×3mm×4 mm (external diameter×length×internal diameter) arealso particularly suitable according to the invention as carriers. Thefineness of the catalytically active oxide materials to be applied tothe surface of the carrier is of course adapted to the desired coatthickness (cf. EP-A 714 700).

Advantageous multimetal oxide active materials to be used according tothe invention are furthermore materials of the formula II

[Y ¹ _(a) ,y ² _(b) ,O _(x),]_(p) [Y ³ _(c) ,Y ⁴ _(d) ,Y ⁵ _(e) ,Y ⁶_(f) ,Y ⁷ _(g) ,Y ² _(h) ,O _(y),]_(q)  (II),

where

Y¹ is bismuth, tellurium, antimony, tin and/or copper,

Y² is molybdenum and/or tungsten,

Y³ is an alkali metal, thallium and/or samarium,

Y⁴ is an alkaline earth metal, nickel, cobalt, copper, manganese, zinc,tin, cadmium and/or mercury,

Y⁵ is iron, chromium, cerium and/or vanadium,

Y⁶ is phosphorus, arsenic, boron and/or antimony, and

Y⁷ is a rare earth metal, titanium, zirconium, niobium, tantalum,rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium,silicon, germanium, lead, thorium and/or uranium,

a′ is from 0.01 to 8,

b′ is from 0.1 to 30,

c′ is from 0 to 4,

d′ is from 0 to 20,

e′ is from 0 to 20,

f′ is from 0 to 6,

g′ is from 0 to 15,

h′ is from 8 to 16,

x′,y′ are numbers which are determined by the valency and frequency ofthe elements in II other than oxygen and

p,q are numbers whose ratio p/q is from 0.1 to 10,

containing three-dimensional regions of the chemical composition Y¹_(a),Y² _(b,)) _(x), which, owing to their composition differing fromtheir local environment, are delimited with respect to their localenvironment and whose maximum diameter (longest distance between twopoints present on the surface (interface) of the region and passingthrough the center of gravity of the region) is from 1 nm to 100 μm,frequently from 10 nm to 500 nm or from 1 μm to 50 or 25 μm.

Particularly advantageous novel multimetal oxide materials II are thosein which Y¹ is bismuth.

Preferred among these in turn are those which are of the formula III

 [Bi _(a″) Z ² _(b″) O _(x″)]_(p″ [) Z ² ₁₂ Z ³ _(c″) Z ⁴ _(d″) Fe _(e″)Z ⁵ _(f″) Z ⁶ _(g″) Z ⁷ _(h″) O _(y″)]_(q″)  (III)

where

Z² is molybdenum and/or tungsten,

Z³ is nickel and/or cobalt,

Z⁴ is thallium, an alkali metal and/or an alkaline earth metal,

Z⁵ is phosphorus, arsenic, boron, antimony, tin, cerium and/or lead,

Z⁶ is silicon, aluminum, titanium and/or zirconium,

Z⁷ is copper, silver and/or gold,

a″ is from 0.1 to 1,

b″ is from 0.2 to 2,

c″ is from 3 to 10,

d″ is from 0.02 to 2,

e″ is from 0.01 to 5, preferably from 0.1 to 3,

f″ is from 0 to 5,

g″ is from 0 to 10,

h″ is from 0 to 1,

x″,y″ are numbers which are determined by the valency and frequency ofthe elements in III other than oxygen and

p″,q″ are numbers whose ratio p″/q″ is from 0.1 to 5, preferably from0.5 to 2,

very particularly preferred materials III being those in which Z² _(b″)is (tungsten)_(b″) and Z² ₁₂ is (molybdenum)₁₂.

It is furthermore advantageous if at least 25 mol % (preferably at least50, particularly preferably at least 100, mol %) of the total amount [Y¹_(a),Y² _(b),O_(x),]_(p) ([Bi_(a″)Z² _(b″)O_(x″)]_(p″)) of themultimetal oxide materials II (multimetal oxide materials III) suitableaccording to the invention are present in the multimetal oxide materialsII (multimetal oxide materials III) suitable according to the inventionin the form of three-dimensional regions of the chemical composition Y¹_(a),Y² _(b),O_(x), [Bi_(a″)Z² _(b″)O_(x″)] which, owing to theirchemical composition differing from their local environment, aredelimited with respect to their local environment and whose maximumdiameter is from 1 nm to 100 μm.

Regarding the shaping, the statements made with respect to the catalystscomprising multimetal oxide materials I are applicable to catalystscomprising multimetal oxide materials II.

The preparation of active materials from multimetal oxide materials IIis described, for example, in EP-A 575897 and in DE-A 19855913.

In a manner expedient in terms of application technology, the novelprocess is carried out in a two-zone tube-bundle reactor. A preferredvariant of a two-zone two-bundle reactor which can be used according tothe invention is disclosed in DE-C 2830765. However, the two-zonetube-bundle reactors disclosed in DE-C 2513405, U.S. Pat. No. 3147084,DE-A 2201528 and DE-A 2903218 are also suitable for carrying out thenovel process.

This means that, in the simplest procedure, the fixed-bed catalyst to beused according to the invention is present in the metal tubes of atube-bundle reactor, and two heating media, as a rule salt melts,essentially spatially separated from one another, are passed around themetal tubes. The tube section over which the respective salt bathextends represents, according to the invention, a reaction zone, i.e. inthe simplest procedure a salt bath A flows around that section of thetubes (the reaction zone A) in which the oxidative reaction of thepropene (in a single pass) takes place until a conversion of from 40 to80 mol % is reached, and a salt bath B flows around that section of thetubes (the reaction zone B) in which the subsequent oxidative reactionof the propene (in a single pass) takes place until a conversion of atleast 90 mol % is reached (if required, further reaction zones which arekept at individual temperatures may follow the reaction zones A, B to beused according to the invention.

It is expedient in terms of application technology if the novel processcomprises no further reaction zone, i.e. the salt bath B expedientlyflows around that section of the tubes in which the subsequent oxidativereaction of the propene (in a single pass) takes place up to aconversion of ≧92 mol % or ≧94 mol % or more.

Usually, the beginning of the reaction zone B is behind the hot-spotmaximum of the reaction zone A. The hot-spot maximum of the reactionzone B is usually below the hot-spot maximum temperature of the reactionzone A.

According to the invention, the two salt baths A, B can be passedcocurrent or countercurrent through the space surrounding the reactiontubes, relative to the direction of flow of the reaction gas mixtureflowing through the reaction tubes. According to the invention, it is ofcourse also possible to employ cocurrent flow in the reaction zone A andcountercurrent flow in the reaction zone B (or vice versa).

In all abovementioned configurations, it is of course also possible tosuperpose a transverse flow on the flow of the salt melt parallel to thereaction tubes, within the respective reaction zone, so that theindividual reaction zone corresponds to a tube-bundle reactor asdescribed in EP-A 700714 or in EP-A 700893 and a meandering flow of theheat exchange medium results through the catalyst tube bundle in thelongitudinal section as a whole.

Expediently, the reaction gas starting mixture is preheated to thereaction temperature before being fed to the catalyst bed.

In the abovementioned tube-bundle reactors, the catalyst tubes areusually produced from ferritic steel and typically have a wall thicknessof from 1 to 3 mm. Their internal diameter is as a rule from 20 to 30mm, frequently from 22 to 26 mm. It is expedient in terms of applicationtechnology if the number of catalyst tubes housed in the tube-bundlecontainer is at least 5000, preferably at least 10,000. Frequently, thenumber of catalyst tubes housed in the reaction container is from 15,000to 30,000. Tube-bundle reactors having more than 40,000 catalyst tubestend to be the exception. Inside the container, the catalyst tubes areusually homogeneously distributed, the distribution expediently beingchosen so that the distance between the central inner axes of adjacentcatalyst tubes (the catalyst tube spacing) is from 35 to 45 mm (cf. forexample EP-B 468290).

Particularly suitable heat exchange media are fluid heating media. Theuse of melts of salts such as potassium nitrate, potassium nitrite,sodium nitrite and/or sodium nitrate or of metals having a low meltingpoint, such as sodium, mercury or alloys of various metals, isparticularly advantageous.

In all abovementioned configurations of the flow in the two-zonetube-bundle reactors, the flow rate inside the two required circulationsof heat exchange medium is as a rule chosen so that the temperature ofthe heat exchange medium increases by from 0 to 15° C. from the point ofentry into the reaction zone to the point of exit from the reactionzone, i.e. the abovementioned ΔT may be, according to the invention,from 1 to 10° C., or from 2 to 8° C. or from 3 to 6° C.

The temperature of the heat exchange medium on entry into the reactionzone A is, according to the invention, usually from 300 to 350° C.According to the invention, the temperature of the heat exchange mediumon entry into the reaction zone B is usually, on the one hand, from 305to 380° C. and, on the other hand, is simultaneously at least 5° C.above the temperature of the heat exchange medium entering the reactionzone A.

The temperature of the heat exchange medium on entry into the reactionzone B is preferably at least 10° C. above the temperature of the heatexchange medium entering the reaction zone A. The difference between thetemperatures on entry into the reaction zones A and B may thus be,according to the invention, up to 20° C., up to 25° C., up to 30° C., upto 40° C., up to 45° C. or up to 50° C. Usually, however, theabovementioned temperature difference is not more than 50° C. The higherthe chosen propene loading of the catalyst bed during the novel process,the greater should be the difference between the temperature of the heatexchange medium on entry into the reaction zone A and the temperature ofthe heat exchange medium on entry into the reaction zone B.

Advantageously, the temperature of the heat exchange medium on entryinto the reaction zone B is, according to the invention, from 305 to365° C. or 340° C., particularly advantageously from 310 to 330° C.

In the novel process, the two reaction zones A, B can of course also berealized in tube-bundle reactors spatialy separated from one another. Ifrequired, a heat exchanger may also be mounted between the two reactionzones. The two reaction zones A, B can of course also be designed as afluidized bed.

In the novel process, it is also possible to use catalyst beds whosevolume-specific activity in the direction of flow of the reactionmixture increases continuously, abruptly or stepwise (this can beeffected as described in WO 98/24746 or in JP-A 91/294239 or by dilutionwith inert material). Moreover, the inert diluent gases (for exampleonly propane or only methane etc.) recommended in EP-A 293224 and inEP-B 257565 can also be used for the two-zone procedure described. Thelatter can, if required, also be combined with a volume-specificactivity of the catalyst bed which increases in the direction of flow ofthe reaction gas mixture.

It should once again be pointed out here that, for carrying out thenovel process, it is also possible to use in particular the two-zonetube-bundle reactor type which is described in German publishedapplication DAS 2,201,528 and includes the possibility of transferring aportion of the hotter heat exchange medium of the reaction zone B to thereaction zone A in order, if required, to heat up a cold reaction gasstarting mixture or a cold recycle gas.

The novel process is particularly suitable for being carried outcontinuously. It is surprising that it permits the higher space-timeyield in the formation of the desired product in a single pass withoutat the same time significantly impairing the selectivity of theformation of the desired product. Rather, a nonsignificantly higherselectivity in the formation of the desired product is even observed.

The latter is presumably due to the fact that, owing to the highertemperatures present in the region of higher propene conversion, thenovel process results in less readsorption of the resulting acroleinonto the fixed-bed catalyst.

It is also noteworthy that the catalyst life in the novel process iscompletely satisfactory in spite of the extreme loading of the catalystwith the reactants.

The novel process does not give pure acrolein but a mixture from whosesecondary components the acrolein can be separated off in a manner knownper se. Unconverted propene and inert diluent gas used and/or formed inthe course of the reaction can be recycled to the gas-phase oxidation.When acrolein is used for the preparation of acrylic acid by two-stagecatalytic gas-phase oxidation of propene, the acrolein-containingreaction gases are transferred to the second oxidation stage as a rulewithout removal of the secondary components. If required, the noveltwo-zone procedure can of course also be used in the case ofconventional propene loads.

Otherwise, conversion, selectivity and residence time are defined asfollows in this publication, unless stated otherwise:${{Conversion}\quad C_{p}\quad {of}\quad {propeno}\quad (\%)} = {\frac{{Number}\quad {of}\quad {moles}\quad {of}\quad {propene}\quad {converted`}}{{Number}\quad {of}\quad {moles}\quad {of}\quad {propene}\quad {used}} \times 100}$$\begin{matrix}{{Selectivity}\quad S_{A}\quad {of}\quad {the}\quad {acrolein}} \\{{formation}\quad (\%)}\end{matrix} = {\frac{\begin{matrix}{{Number}\quad {of}\quad {moles}\quad {of}\quad {propene}} \\{{converted}\quad {into}\quad {acrolein}}\end{matrix}}{{Number}\quad {of}\quad {moles}\quad {of}\quad {propene}\quad {converted}} \times 100}$$\begin{matrix}{{Selectivity}\quad S_{AS}\quad {of}\quad {the}\quad {formation}\quad {of}} \\{{acrylic}\quad {acid}\quad {byproduct}\quad (\%)}\end{matrix} = {\frac{\begin{matrix}{{Number}\quad {of}\quad {moles}\quad {of}\quad {propene}} \\{{converted}\quad {into}\quad {acrylic}\quad {acid}}\end{matrix}}{{Number}\quad {of}\quad {moles}\quad {of}\quad {propene}\quad {converted}} \times 100}$$\begin{matrix}{{Selectivity}\quad S_{DP}\quad {of}\quad {the}\quad {formation}\quad {of}\quad {the}} \\{{desired}\quad {product}\quad (\%)}\end{matrix} = {\frac{\begin{matrix}{{Number}\quad {of}\quad {moles}\quad {of}\quad {propene}\quad {converted}} \\{{into}\quad {acrolein}\quad {and}\quad {into}\quad {acrylic}\quad {acid}}\end{matrix}}{{Number}\quad {of}\quad {moles}\quad {of}\quad {propene}\quad {converted}} \times 100}$${{Residence}\quad {time}\quad ( \sec )} = {\frac{{Empty}\quad {reactor}\quad {volume}\quad {filled}\quad {with}\quad {catalyst}\quad (1)}{{Throughput}\quad {of}\quad {reaction}\quad {gas}\quad {starting}\quad {mixture}\quad ( {1/h} )} \times 3600}$

EXAMPLES a) Catalyst Preparation

1. Preparation of a Starting Material 1

209.3 kg of tungstic acid (72.94% by weight of W) were stirred, a littleat a time at 25° C., into 775 kg of an aqueous solution of bismuthnitrate in nitric acid (11.2% by weight of Bi, free nitric acid from 3to 5% by weight; density: from 1.22 to 1.27 g/ml). The resulting aqueousmixture was then stirred for a further 2 hours at 25° C. and thenspray-dried.

The spray-drying was carried out in a rotating-disk spray tower by thecountercurrent method at a gas inlet temperature of 300±10° C. and a gasoutlet temperature of 100±10° C. The spray-dried powder obtained wasthen calcined at from 780 to 810° C. (in a rotary tubular furnacethrough which air flowed (1.54 m³ internal volume, 200 m³ (S.T.P.) ofair/h)). When establishing the exact calcination temperature, it isimportant that it is tailored to the desired phase composition of thecalcination product. The phases WO₃ (monoclinic) and Bi₂W₂O₉ aredesired, and the presence of γ-Bi₂WO₆ (russellite) is undesired. If,therefore, the compound γ-Bi₂WO₆ is still detectable after calcinationon the basis of reflection at an angle of 2⊖=28.4° (CuKα radiation) inthe powder X-ray diffraction pattern, the preparation should be repeatedand the calcination temperature increased within the stated temperaturerange until the reflection disappears. The preformed calcined mixedoxide obtained in this manner was milled so that the X₅₀ value (cf.Ullmann's Encyclopedia of Industrial Chemistry, 6th Edition (1998)Electronic Release, section 3.1.4 or DIN 66141) of the resultingparticle size distribution was 5 μm. The milled material was then mixedwith 1% by weight (based on the milled material) of finely divided SiO₂(bulk density 150 g/l; X₅₀ value of the SiO₂ particles was 10 μm, theBET surface area was 100 m²/g).

2. Preparation of a Starting Material 2

A solution A was prepared by dissolving 213 kg of ammoniumheptamolybdate in 600 1 of water at 60° C. while stirring, and adding0.97 kg of an aqueous potassium hydroxide solution (46.8% by weight ofKOH) at 20° C. to the resulting solution while maintaining the 60° C.and stirring.

A solution B was prepared by introducing 116.25 kg of an aqueous ironnitrate solution (14.2% by weight of Fe) into 262.9 kg of an aqueouscobalt nitrate solution (12.4% by weight of Co) at 60° C. The solution Bwas then pumped continuously over a period of 30 minutes into theinitially taken solution A while maintaining the 60° C. Stirring wasthen carried out for 15 minutes at 60° C. Thereafter, 19.16 kg of asilica gel (46.80% by weight of SiO₂, density: from 1.36 to 1.42 g/ml,pH from 8.5 to 9.5, alkali metal content not more than 0.5% by weight)were added to the resulting aqueous mixture and stirring was thencarried out for a further 15 minutes at 60° C.

Spray drying was then carried out in a rotating-disk spray tower by thecountercurrent method (gas inlet temperature: 400±10° C., gas outlettemperature: 140±5° C.). The resulting spray-dried powder had a loss onignition of about 30% by weight (calcining for 3 hours at 600° C.).

3. Preparation of the Multimetal Oxide Active Material

The starting material 1 was homogeneously mixed with the startingmaterial 2 in the amount required for a multimetal oxide active materialhaving a stoichiometry

[Bi ₂ W ₂ O _(9 .)2WO ₃]_(0.5) [Mo ₁₂ Co _(5.5) Fe _(2.94) Si _(1.59) K_(0.08) O _(x)]1

In addition, 1.5% by weight, based on the abovementioned total material,of finely divided graphite (sieve analysis: min. 50% by weight <24 μm.max. 10% by weight >24 μm and <48 μm, max. 5% by weight <48 μm, BETsurface area: from 6 to 13 m²/g) were mixed in homogeneously. Theresulting dry blend was molded to give hollow cylinders having a lengthof 3 mm, an external diameter of 5 mm and a wall thickness of 1.5 mm andthen subjected to a thermal treatment as follows.

In a muffle furnace through which air flowed (60 l internal volume, 1l/h of air per gram of active precursor material), heating was effectedat a heating rate of 180° C./h, initially from room temperature (25° C.)to 190° C. This temperature was maintained for 1 hour and then increasedto 210° C. at a heating rate of 60° C./h. The 210° C. were in turnmaintained for 1 hour before being increased at a heating rate of 60°C./h, to 230° C. This temperature was likewise maintained for 1 hourbefore it was increased to 265° C., once again at a heating rate of 60°C./h. The 265° C. were then likewise maintained for 1 hour. Thereafter,cooling to room temperature was initially effected, thus essentiallycompleting the decomposition phase. Thereafter, heating was effected to465° C. at a heating rate of 180° C./h, and this calcination temperaturewas maintained for 4 hours.

The resulting unsupported catalyst rings were used for the catalyticgas-phase propene oxidation described below.

b) Gas-phase Catalytic Oxidation of Propene to Acrolein

1. Loading of the Reaction Tube

A reaction tube (V2A stainless steel; 30 mm external diameter; 2 mm wallthickness; 26 mm internal diameter; length: 439 cm, having a tube (4 mmexternal diameter) centered in the center of the reaction tube andsuitable for holding a thermocouple with which the temperature in thereaction tube could be determined) was loaded, on a catalyst supportledge (44 cm long), from bottom to top, initially over a length of 30 cmwith steatite beads having a rough surface (from 4 to 5 mm diameter;inert material for heating the reaction gas starting mixture) and thenover a length of 300 cm with the unsupported catalyst rings prepared ina), before the loading was completed over a length of 30 cm with theabovementioned steatite beads as a subsequent bed. The remaining 35 cmof catalyst tube were left empty.

That part of the reaction tube which had been loaded with solid wasthemostated by means of 12 aluminum blocks each 30 cm long, which werecast cylindrically around the tube and were heated by electric heatingtapes (comparative experiments using a corresponding reaction tubeheated by means of a salt bath through which nitrogen was bubbled showedthat the thermostating by means of aluminum blocks was capable ofsimulating thermostating by means of a salt bath. The first six aluminumblocks in the direction of flow defined a reaction zone A and theremaining six aluminum blocks defined a reaction zone B. Those ends ofthe reaction tube which were free of solid were kept at 220° C. by meansof steam under superatmospheric pressure.

The reaction tube described above was fed continuously with a reactiongas starting mixture of the following composition, the loading and thethermostating of the reaction tube being varied:

from 6 to 6.5% by volume of propene,

from 3 to 3.5% by volume of H_(2O,)

from 0.3 to 0.5% by volume of CO,

from 0.8 to 1.2% by volume of CO₂,

from 0.025 to 0.04% by volume of acrolein,

from 10.4 to 10.7% by volume of O₂, the remaining amount to 100% being

molecular nitrogen (oxygen source was air, apart from a low O₂ contentof the recycle gas)

A small sample for gas chromatographic analysis was taken from theproduct gas mixture at the reaction tube exit. Otherwise, the productgas mixture was fed directly into a downstream acrolein oxidation stage(to give acrylic acid). The acrylic acid was separated from the productgas mixture of the acrolein oxidation stage in a manner known per se,and a part of the remaining residual gas was reused for feeding thepropene oxidation stage (as recycle gas), which explains the acroleincontent of the abovementioned feed gas and the small variation in thefeed composition.

The pressure at the reaction tube entrance varied, as a function of thechosen propene loading, in the range from 3.0 to 1.9 bar. An analysispoint was likewise present at the end of the reaction zone A.

The results obtained depending on the chosen propene loading and on thechosen aluminum thermostating are shown in Table 1 below.

T_(A) is the temperature of the aluminum blocks in the reaction zone Aand T_(B) is the temperature of the aluminum blocks in the reaction zoneB.

U_(PA) is the propene conversion at the end of the reaction zone A andCPE is the propene conversion at the reaction tube exit. S_(AE), S_(AAE)and S_(DPE) are the selectivities S_(A), S_(AA) and S_(DP) at thereaction tube exit and STY_(DP) is the space time yield of the desiredproduct at the reaction tube exit.

TABLE 1 Propene loading STY_(DP) [l(S.T.P.) of propene/l · h] T_(A) [°C.] T_(B) [° C.] C_(PA) (%) C_(PE) (%) S_(AE) (%) S_(AAE) (%) S_(DPE)(%) (g/l · h) 100 312 312 76.9 94.6 91.82 4.44 96.3 230.8 125 316 31678.2 94.1 92.73 5.09 97.8 292.2 175 325 336 76.3 94.9 90.07 7.34 97.4413.5 175 320 341 72.1 95.0 90.13 7.24 97.4 474.2 200 325 346 73.7 94.590.65 7.49 98.1 468.8

If the propene loading is increased <200 l(S.T.P.) of propene/l·h, theresults shown in Table 2 are obtained. Table 2 additionally shows theconditions to be established for a comparative experiment at 200° C.

TABLE 2 Propene loading STY_(DP) [l(S.T.P.) of propene/l · h] T_(A) [°C.] T_(B) [° C.] C_(PA) (%) C_(PE) (%) S_(AE) (%) S_(AAE) (%) S_(DPE)(%) (g/l · h) 225 325 357 72.4 94.5 89.22 7.88 97.1 528.6 250 330 36172.3 94.5 88.84 8.07 96.91 586.5 200 (Comparative experiment) 300 370 4494.5 88.2 8.5 96.7 468.8

We claim:
 1. A process for the catalytic gas-phase oxidation of propeneto acrolein, in which a reaction gas starting mixture comprisingpropene, molecular oxygen and at least one inert gas, at least 20% byvolume of which consists of molecular nitrogen, and containing themolecular oxygen and the propene in a molar ratio O₂:C₃H₆ of ≧1 ispassed, at elevated temperatures, over a fixed-bed catalyst, whoseactive material is at least one molybdenum- and/or tungsten- andbismuth-, tellurium-, antimony-, tin- and/or copper-containingmultimetal oxide, in such a way that the propene conversion in a singlepass is ≧90 mol % and the associated selectivity of the acroleinformation and of the acrylic acid byproduct formation together is ≧90mol %, wherein a) the loading of the fixed-bed catalyst with the propenecontained in the reaction gas starting mixture is ≧160 l(S.T.P.) ofpropene per l of catalyst bed per h, b) the fixed-bed catalyst consistsof a catalyst bed arranged in two spatially successive reaction zones A,B, the temperature of the reaction zone A being from 300 to 390° C. andthe temperature of the reaction zone B being from 305 to 420° C. and atthe same time being at least 50° C. above the temperature of thereaction zone A, c) the reaction gas starting mixture flows firstthrough the reaction zone A and then through the reaction zone B and d)the reaction zone A extends to a propene conversion of from 40 to 80 mol%.
 2. A process as claimed in claim 1, wherein the reaction zone Aextends to a propene conversion of from 50 to 70 mol %.
 3. A process asclaimed in claim 1, wherein the reaction zone A extends to a propeneconversion of from 65 to 75 mol %.
 4. A process as claimed in claim 1,wherein the temperature of the reaction zone B is at least 10° C. abovethe temperature of the reaction zone A.
 5. A process as claimed in 1,wherein the temperature of the reaction zone B is from 305 to 340° C. 6.A process as claimed in claim 1, wherein the temperature of the reactionzone B is from 310 to 330° C.
 7. A process as claimed in claim 1,wherein the propene conversion in a single pass is ≧94 mol %.
 8. Aprocess as claimed in claim 1, wherein the selectivity of the acroleinformation and of the formation of acrylic acid byproduct together is ≧94mol %.
 9. A process as claimed in claim 1, wherein the propene loadingof the catalyst bed is ≧165 l (S.T.P.)/l·h.
 10. A process as claimed inclaim 1, wherein the propene loading of the catalyst bed is ≧170 l(S.T.P.)/l·h.
 11. A process as claimed in claim 1, wherein the one ormore inert gases comprise ≧40% by volume of molecular nitrogen.
 12. Aprocess as claimed in claim 1, wherein the one or more inert gasescomprise ≧60% by volume of molecular nitrogen.
 13. A process as claimedin claim 1, wherein the one or more inert gases comprise steam.
 14. Aprocess as claimed in claim 1, wherein the one or more inert gasescomprise CO₂ and/or CO.
 15. A process as claimed in claim 1, which iscarried out at an operating pressure of from 0.5 to 3.5 bar.
 16. Aprocess as claimed in claim 1, wherein the molar O₂:C₃H₆ ratio in thereaction gas starting mixture is from 1.5 to 2.0.
 17. A process asclaimed in claim 1, wherein air is concomitantly used as the oxygensource.
 18. A process as claimed in claim 1, wherein the propene contentof the reaction gas starting mixture is from 4 to 15% by volume.
 19. Aprocess as claimed in claim 1, wherein the propene content of thereaction gas starting mixture is from 5 to 12% by volume.
 20. A processas claimed in claim 1, wherein the propene content of the reaction gasstarting mixture is from 5 to 8% by volume.
 21. A process as claimed inclaim 1, wherein the active material of the fixed-bed catalyst is atleast one multimetal oxide of the formula I Mo₁₂Bi_(a)Fe_(b)X¹ _(c)X²_(d)X³ _(e)X⁴ _(f)O_(n)  (1) where X¹ is nickel and/or cobalt, X² isthallium, an alkali metal and/or an alkaline earth metal, X³ is zinc,phosphorus, arsenic, boron, antimony, tin, cerium, lead and/or tungsten,X⁴ is silicon, aluminum, titanium and/or zirconium, a is from 0.5 to 5,b is from 0.01 to 5, c is from 0 to 10, d is from 0 to 2, e is from 0 to8, f is from 0 to 10 and n is a number which is determined by thevalency and frequency of the elements in I other than oxygen.
 22. Aprocess as claimed in claim 1, wherein the active material of thefixed-bed catalyst is at least one multimetal oxide of the formula II [Y¹ _(a′)Y² _(b′)O_(x′)]_(p)[Y³ _(c′)Y⁴ _(d′)Y⁵ _(e′)Y⁶ _(f′)Y⁷ _(g′)Y²_(h′)Y⁷ _(g′)Y² _(h′)O⁷ _(y′)]_(q)  (II), where Y¹ is bismuth,tellurium, antimony, tin and/or copper, Y² is molybdenum and/ortungsten, Y³ is an alkali metal, thallium and/or samarium, Y⁴ is analkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin,cadmium and/or mercury, Y⁵ is iron, chromium, cerium and/or vanadium, Y⁶is phosphorus, arsenic, boron and/or antimony, Y⁷ is a rare earth metal,titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium,silver, gold, aluminum, gallium, indium, silicon, germanium, lead,thorium and/or uranium, a′ is from 0.01 to 8, b′ is from 0.1 to 30, c′is from 0 to 4, d′ is from 0 to 20, e′ is from 0 to 20, f′ is from 0 to6, g′ is from 0 to 15, h′ is from 8 to 16, x′ and y′ are numbersdetermined by the valency and frequency of the elements in II other thanoxygen and p and q are numbers whose ratio p/q is from 0.1 to 10,containing three-dimensional regions of the chemical composition Y¹_(a), Y² _(b), O_(x), which are delimited from their local environmentowing to their composition differing from their local environment andwhose maximum diameters are from 1 nm to 100 μm.
 23. A process asclaimed in claim 1, wherein the catalyst bed comprises annular and/orspherical catalysts.
 24. A process as claimed in claim 23, wherein thering geometry is the following: external diameter: from 2 to 10 mm,length: from 2 to 10 mm, wall thickness: from 1 to 3 mm.
 25. A processas claimed in claim 23, wherein the spherical catalyst is a coatedcatalyst consisting of a spherical carrier (from 1 to 8 mm diameter) anda coat (from 10 to 1000 μm thick) comprising active material and appliedto said carrier.
 26. A process as claimed in claim 1, which is carriedout in a two-zone tube-bundle reactor.
 27. A process for the preparationof acrylic acid, which comprises a process as claimed in claim 1.